The present invention relates to a nonlinear distortion compensating technique in a transmission apparatus for digital radio communications.
In recent years, a mobile communication system using a digital modulation method has vigorously been researched and developed. If a high efficiency amplifier is employed in the transmission system in order to attempt to save power at a radio transceiver, nonlinear distortions frequently result therefrom. As a means, there is a method for compensating nonlinear distortions of amplitude and phase with reference to a distortion compensating table, by using an amplitude value of transmission base band signals.
A description is given of a conventional transmission apparatus which compensates nonlinear distortions by using such a method.
FIG. 10 is a main block diagram of a conventional transmission apparatus. In FIG. 10, amplitude calculating section 1001 calculates amplitude information 1010 of transmission signals 1009 (transmission digital quadrature base band of I and Q channels), and compensation table 1002 outputs distortion compensating coefficient 1011. The distortion compensating section 1003 outputs a distortion compensating signal 1012 according to the transmission signal 1009 and distortion compensating coefficient 1011. The outputted distortion compensation signal 1012 is quadrature-modulated by quadrature modulating section 1004, and the modulated signal 1013 is amplified by amplifier 1005, wherein amplified RF signal 1014 is outputted.
Furthermore, demodulating section 1007 demodulates feedback RF signal 1016 fed back from coupler 1006 to feedback base band signal 1017, and estimating section 1008 updates the distortion compensation coefficient of compensation table 1002 on the basis of a distortion compensation coefficient 1018, transmission signal 1009 and feedback base band signal 1017. Through the abovementioned actions, amplified RF signal 1015 for which nonlinear distortion compensation is carried out is outputted from the coupler 1006.
In addition thereto, as a method to compensate nonlinear distortions in the transmission system, there is a method having a digital filter to which a ROM (Read-only-memory) is attached, wherein compensation is carried out by applying in advance distortions to compensate nonlinear distortions generated in an amplifier by the digital filter.
The main block diagram of a conventional transmission apparatus in which this method is employed is shown in FIG. 11. Hereinafter, a description is given to this apparatus.
In FIG. 11, digital filter 1102 in which digital signals 1101 is inputted gives to the digital signal 1101 distortions to compensate nonlinear distortion components generated in radio frequency power amplifier 1105, by using distortion information stored in a ROM in advance.
The digital signal 1101 to which distortion to be compensated is given is digital-analog converted and modulated by quadrature modulating section 1103 and inputted into the radio frequency power amplifier 1105 via transmission section 1104. In the radio frequency power amplifier 1105, since distortion of the inputted digital signal are compensated in advance, distortions generated at the radio frequency power amplifier 1105 are cancelled by those for compensation.
Furthermore, instead of ROM, there is still another method to compensate nonlinear distortions, in which a RAM (Random Access Memory) having compensation coefficients stored therein in order to compensate nonlinear distortion components is used, by varying the compensation coefficients of the RAM in compliance with amplitudes of digital signals.
Furthermore, Unexamined Japanese Patent Publication No. 290321 of 1992 discloses a method for controlling actions of a digital filter by feeding outputs of a radio frequency power amplifier back to the digital filter.
However, in the conventional example shown in FIG. 10, it is necessary that signals having the maximum amplitude is suppressed less than the maximum output of amplifier 1005, and this results in a lowering of efficiency in the amplifier 1005.
In the conventional example shown in FIG. 11, since it is necessary to provide a memory table such as a ROM or RAM in which compensation coefficients are stored to compensate nonlinear distortion components, and this results in an increase of the scale of transmission circuits themselves.
It is therefore an object of this invention to provide a transmission apparatus capable of easily controlling leak power and easily improving the efficiency of power amplification and cable of compensating nonlinear distortions generated in a transmission system amplifier without any use of a memory table such as a ROM or RAM.
The first aspect of the invention resides in a transmission apparatus having a nonlinear distortion compensating circuit, which includes an amplitude limiting function for transmission quadrature base band signals by adding an amplitude calculating section, an amplitude limiting table and an amplitude limiting section thereto. Thereby, it is possible to easily improve the efficiency of amplification section with distortions of the entire system limited.
The second aspect of the invention is such that nonlinear distortion compensation is carried out on the basis of nonlinear distortion compensation coefficients calculated by an approximation equation in a compensation coefficient calculating section. Thereby, it is possible to compensate nonlinear distortions generated by amplifier in the transmission system without any use of a memory table such as a ROM or RAM, and it is possible to make a nonlinear distortion compensating section small-sized.
Furthermore, this invention is constructed so that it is provided with a first amplitude calculating section for calculating the first amplitude value of transmission quadrature base band signals; an amplitude limiting table for storing amplitude limiting information corresponding to the first amplitude value; a first amplitude limiting section for limiting the amplitude of the transmission quadrature base band signals by using the amplitude limiting information; a quadrature modulating section for outputting RF signals by quadrature-modulating the transmission quadrature base band signals, the amplitude of which is limited; and an amplification section for amplifying the RF signals.
With this construction, it is possible to improve the efficiency of an amplifying section with distortion components limited, by executing distortion compensation of an amplifier with respect to limited signals, the maximum amplitude of which is distorted by the transmission quadrature base band signals.
Furthermore, the invention is constructed so that it is provided with a first amplitude calculating section for calculating the first amplitude value from transmission quadrature base band signals; a limiting coefficient calculating section for calculating an amplitude limiting coefficient corresponding to the first amplitude value; a second amplitude limiting section for limiting the amplitude of the transmission quadrature base band signal by using the amplitude limiting coefficient; a quadrature modulating section for quadrature-modulating the transmission quadrature base band signals to output RF signals , the amplitude of which is limited; and an amplification section for amplifying the RF signals.
With this construction, the amplitude limiting coefficient is calculated on the basis of amplitude information of the transmission quadrature base band signals, whereby it is possible to improve the efficiency of amplifier by limiting the amplitude of transmission signals in compliance with the amplitude limiting coefficient with the distortion components limited, without adding any memory thereto.
Furthermore, this invention is constructed so that it is provided with a power calculating section for calculating a power value of transmission quadrature base band signals; a compensation coefficient calculating section for calculating a nonlinear distortion compensation coefficient according to approximation equation set in advance using the power value; a distortion compensating section for carrying out nonlinear distortion compensation of the transmission quadrature base band signals by using the nonlinear distortion compensation coefficient; a quadrature modulating section for quadrature-modulating transmission quadrature base band signals, the distortion of which is compensated; and an amplifier for amplifying the quadrature modulation signal.
With this construction, with only slight memory capacity, it is possible to compensate nonlinear distortion which is generated at an amplifier of transmission system.
Furthermore, the invention is constructed so that it is provided with a power calculating section for calculating a power value of transmission quadrature base band signals; a compensation coefficient calculating section for calculating an amplitude distortion compensation coefficient by an approximation equation set in advance by using the power value; a quadrature modulating section for quadrature-modulating the transmission quadrature base band signals; and an amplitude distortion compensating section for compensating amplitude distortions of quadrature modulation signals using the amplitude distortion compensation coefficient.
With this construction, it is possible to compensate amplitude distortions, which is generated at an amplifier of transmission system, with simple calculations and slight memory capacity.
Furthermore, the invention is constructed so that it is provided with a power calculating section for calculating a power value of transmission quadrature base band signals; a compensation coefficient calculating section for calculating an amplitude distortion compensation coefficient by an approximation equation set in advance by using the power value; a quadrature modulating section for quadrature-modulating the transmission quadrature base band signals; an amplitude distortion compensating section for compensating amplitude distortions of quadrature modulation signals by using the amplitude distortion compensating coefficient; an amplifier for amplifying modulation signals; a distributor for distributing output of the amplifier; a quadrature demodulator for carrying out quadrature demodulate with one of the outputs of the distributor inputted; and a coefficient updating section which calculates an error by comparing quadrature demodulate signals with the power value and updates the value of coefficients of the approximation equation on the basis of the error.
With this construction, it is possible to very accurately compensate amplitude distortions by lowering the error of amplitude distortion compensation data calculated by an approximation equation through a feedback loop.